In recent years, as the levels of integration and capacity of large scale integrated circuits (LSIs) have increased, there has been a need to continue to reduce the width of the circuit patterns of semiconductor units. Semiconductor units are manufactured by a reduced projection exposure apparatus called a “stepper” using original artwork patterns with a circuit pattern formed thereon, these are called masks or reticles (hereinafter referred to collectively as masks). Specifically, a pattern on a mask is transferred to a wafer by exposure to light, thereby forming circuits on to the wafer. Masks used to transfer such fine circuit patterns to the wafer are manufactured by electron beam writing apparatuses, which can write micropatterns. Further, effort has been made to develop a laser beam writing apparatus, which uses a laser beam for writing. It should be noted that electron beam apparatuses are also used to directly write a circuit pattern on a wafer.
Incidentally, since the cost of manufacturing LSIs is very high, an improvement of yield is required to make the manufacture economically feasible. However, the dimensions of the patterns for LSI units, as typified by 1-gigabit class DRAMs (random access memories), are about to be scaled down from the order of submicrons to the order of nanometers. A major cause of loss in yield is due to defects of a mask pattern. Further, since there has been a decrease in the dimensions of LSI patterns formed on semiconductor wafers, the size of pattern defects to be detected is very small. Therefore, high inspection accuracy is required of mask inspection systems for detecting defects of transfer masks used in LSI manufacture.
One of the methods used for detecting defects is the die-to-database inspection method. In this method, an optical image of a pattern is compared with a reference image of the pattern made from design data inputted into an inspection apparatus. The design data is data converted from CAD data into a format that can be input into the inspection apparatus.
In a die-to-database inspection method, light is emitted from a light source, and the mask to be inspected is irradiated with this light through an optical system. The mask is mounted on a table, and this table is moved so that the emitted beam of light scans the surface of the mask. Light transmitted through or reflected from the mask is focused on an image sensor forming an image thereon. The optical image thus formed on the image sensor is sent to a comparing unit. The comparing unit compares the optical image measurement data with the reference image data in accordance with an appropriate algorithm, and if they are not identical, the mask is determined to have a defect (see Patent Document 1).
[Patent Document 1] Japanese laid-open Patent publication No. 2008-112178
In the defective inspection process, defects are displayed on a monitor based on the data created from the inspection result. The operator determines whether these defects are problematic and classifies the defects accordingly. More specifically, a comparison image is generated from the optical image and the reference image, and then the defects displayed in the comparison image are reviewed by the operator.
In the defective inspection process there is a possibility that a false defect is detected by the inspection apparatus. This false defect can extend the review time for the operator, it is therefore preferred to minimize the occurrence of these false defects.
One example of a false defect is white spot. This false defect is caused by cosmic rays.
An image sensor used on an inspection process is sensitive to the cosmic rays. As a few thousand electron-hole pairs are generated by one cosmic ray, multiple false defects occur in a position where a cosmic ray passes through an optical image. This image is referred to as a white spot, because it is brighter than the adjacent to the optical image. White spot can be confused with a true defect and therefore the white spot should be removed from the inspection result. It is difficult to shield the cosmic rays by use of, for example, a screen.
White spots are also known in astronomy. In astronomy white spot can be confused with a star, as one example. The white spot generates randomly in time and space, but the possibly that a white spot is generated several times at the same position is extremely low. Therefore in astronomy, images of the same object are acquired several times and then are compared with each other, thus the white spot can be removed.
However, it is difficult to apply the above method in an astronomy process to a white spot generated in the inspection process. The time taken for an inspection process is longer than the time taken to acquire information of a star, as one example. The work required for comparing images acquired by repeating the inspection process several times causes a significant decline in productivity. Further, there must be a limit to the amount of defect determination that can be correctly determined by an algorithm for defect inspection, that is, whether a defect is a false defect or not.
The present invention has been conceived in view of the above problem. Therefore, an object of this invention is to provide an inspection apparatus and an inspection method capable of improving the accuracy of the inspection process by removing white spot.
Other challenges and advantages of the present invention are apparent from the following description.